so i want to cut 15% body fat to 10% and want to save as much muscle and strength i possible can but also the fasted way. (who doesn’t)

But i still guessing what’s the best approach for me:

Let me explain:.
so i have learnt when you follow a keto diet, it takes 8-12 weeks when the body is fully adapts and then get all the benefits from being in ketosis.
So that means that after you are fully adapt, then starts the protein sparring effect? and the body is using 100% body-fat for fuel?
Not earlier?
so yes, then is the best approach to first follow 8-12 weeks keto on maintenance kcal or little bulk so that the body not using muscle and protein for fuel and after fully adaptation. when the body is used to burn bodyfat as fuel and not protein and muscle any-more, starting cutting kcal below?

another thought
if its really to take 8 - 12 weeks for fully adapt then is maybe better to forget the keto diet and go for RFL because you can eat tons of protein and don’t care about ketosis,
what’s also limited muscle and strength breakdown and so i can begin immediately to cut down kcal and dont care about addaption.

It stays the same. Protein in RFL only changes with LBM, BF category, activity level.

Here is what i read in "The Ketogenic Diet" , which is correct ?

Assuming zero carbohydrate intake, during the first 3 weeks of a ketogenic diet a protein intake of ~150 grams per day should be sufficient to achieve nitrogen balance. Therefore, regardless of bodyweight, the minimum amount of protein which should be consumed during the initial three weeks of a ketogenic diet is 150 grams per day.

After 3 weeks of ketosis, as little as 50 grams of protein per day should provide enough glucose to achieve nitrogen balance.

Presumably, you've also never used this set up for a keto diet before? What's happening now matters more. So long as you did your calculations correctly and stick to the diet as laid out, you'll drop fat.